The present invention relates to improved alkaline earth-containing structures, such as particulate catalysts, monoliths or contact solids. The alkaline earth metals are divalent Group IIA elements (IUPAC Periodic Table), including calcium, barium, magnesium, beryhium and strontium, and form stable oxides which are often present as hydrates and carbonates.
Various alkaline earth materials, such as oxides, hydroxides and/or carbonates of Group IIA metals, clays, natural or synthetic hydrotalcites ("HTC"), or layered double hydroxides, are useful as contact solids in numerous industrial processes. The layered alkaline earth oxide/hydroxide materials are of particular interest. Recent investigations of hydrotalcite-like materials have provided insight into the varied compositions and crystalline nature of HTC's layered structures. It is generally known that such materials can be synthesized in a platy crystalline form having alternating layers of charged metal oxides and anionic "gallery" layers. X-ray diffraction studies indicate that the d-spacing for natural and synthetic HTC is about 7.5-8.5 Angstroms (.ANG.), depending on the anionic layer components. Substitution of a large variety of anionic species in the hydroxide `gallery` has been shown by prior workers. For instance, borate, carbonate, nitrate, phosphate, sulfite/sulfate, and carboxylate anions are known gallery replacements for hydroxyls in the anionic layers. Pillaring of these layered HTC materials is also known.
Naturally occurring hydrotalcite has the formula: Mg.sub.6 Al.sub.2 (CO.sub.3) (OH).sub.16.4H.sub.2 O, and occurs as a trigonal mineral. A structural description of the class of "brucite" crystals is found in the work of Pinnavaia et al (U.S. Pat. No 5,358,701, incorporated by reference). Briefly, the metal oxide layers consist essentially of divalent alkaline earth oxide units, such as magnesium oxide (MgO), configured structurally with octahedral hydroxy groups. A trivalent metal oxide, such as alumina, can be inserted into the brucite crystalline lattice in the octahedrally-coordinated metal oxide layer. In the case of natural hydrotalcite, a carbonate layer may be substituted therein.
In view of a limited supply of naturally-occurring hydrotalcite, various efforts have been made to achieve a reasonably economic alternative material in the form of synthetic `Layered Double Hydroxides. Production of analogous layered crystalline materials has been fruitful and it may be expected that important new uses for the class of hydrotalcite-like crystalline solids will increase its industrial demand.